A fingerprint database generally lists many fingerprint images with, for each one, a set of characteristic points and a match with the identity of the person to whom the fingerprint is assigned.
A fingerprint image consists of a number of lines of varying darkness separated by lines of varying lightness with line ends and line bifurcations which form the characteristic points of the fingerprint.
In order to establish a one-to-one match between an image which results from a print mark of unknown identity and a print image from the database, the method is carried out overall as follows. The unknown print image is presented to an expert who determines characteristic points of the unknown print. The unknown print image with its specified characteristic points is submitted to a computer system which compares it with the images from the database, with their characteristic points. From among the images in the database, the computer system selects a sample of those that are considered, in a known manner, by an algorithm to be closest to the submitted image. The images selected by the computer system are then presented to the expert who compares them with the unknown print image so as to find a match of the unknown print image with a print image from the database.
On an image, available with various levels in a grey scale in order to show the ridges of a fingerprint, the distribution between light lines and dark lines is sometimes difficult to assess. So as to relieve the work of the expert when sorting through the images of a sample put forward, the computer system is generally designed to select a sample containing a limited number of images.
To do this, some images are rejected as soon as their similarity with the submitted image differs, even a little, therefrom. This is a drawback since there is a risk that relevant images are rejected, with the risk that the fingerprint image which actually matches that of the submitted image is not presented to the expert. The expert then spends needless energy observing a sample which does not contain the image sought.
On the other hand, the difficulty in accurately assessing the passage from a dark line to a light line may generate errors in detection and/or positioning of characteristic points on the unknown print image. Since the computer system then compares characteristic points of the unknown print with those of a database, there is a risk of rejecting a relevant database print because of these errors. Before submitting the fingerprint image with its characteristic points to the computer system, the expert must therefore pay close attention to analysing the unknown print in order to determine the characteristic points, thereby minimizing the risk of errors.
Increasing the size of the sample with fewer comparison restrictions decreases the risk of rejecting the relevant image. However, scrutiny of a considerably high number of images displayed with levels from a grey scale increases the fatigue of the expert and risks decreasing his productivity.